Scanning-type sputtering mass spectrometer

ABSTRACT

A scanning-type sputtering mass spectrometer comprising a primary ion-scanning unit including electrostatic-deflecting means and a secondary ion-analyzing unit including electrostaticaccelerating means, elecromagnetic-deflecting means and an ion collector. The accelerating voltage or the deflecting magnetic flux density is controlled in magnitude in synchronism with the scanning of the primary ion beam so as to let only ions of a particular mass-to-change ratio into the ion collector, irrespective of the displacement of the subject point of the secondary ion-analyzing unit.

United States Patent [72] Inventors Hiroshi Doi Ome;

Hiiumi Tamura, l'lachioji; ltiro Omura, Hino, all of Japan 53,481

July 9, 1970 Dec. 14, 1971 Hitachi, Ltd. Tokyo, Japan July 1 1, 1969 Japan Appl. No. Filed Patented Assignee Priority SCANNING-TYPE SPUTTERING MASS SPECTROMETER 5 Claims, 16 Drawing Figs.

US. Cl 250/4l.9 ME, 250/4 1 .9 SE, 250/49.5 P Int. Cl H0 lj 39/36 Field of Search 250/49.5 P,

[56] References Cited UNITED STATES PATENTS 3,517,191 6/1970 Liebl 250/49.5 P

OTHER REFERENCES Long; Brit. Journal Appl. Phys, 1965, Vol. 16, pp. 1,277- l,284; 250- 49.5 P

Primary Examiner-Anthony L. Birch Anorney-Craig, Antonelli and Hill ABSTRACT: A scanning-type sputtering mass spectrometer comprising a primary ion-scanning umit including electrostatic-deflecting means and a secondary ion-analyzing unit including electrostatic-accelerating means, elecromagneticdeflecting means and an ion collector. The accelerating voltage or the deflecting magnetic flux density is controlled in magnitude in synchronism with the scanning of the primary ion beam so as to let only ions of a particular muss-to-change ratio into the ion collector, irrespective of the displacement of the subject point ofthe secondary ion-analyzing unit.

PATENTEDUEBWQYI 3628mm SHEEI 1 BF 2 F/Gz RR/MARY CURRE/vr x /v SCURCE LCE/VERATCR A 1 A] i /,4 SCANNING 2 VOLTAGE SOURCE 6 CURRE/vr CCURCE VOLT/16E [5 SOURCE A l F/Gf 2 UXMX) XLL;

INVENTORS HIRO HI DOI, Hmml TAMURA AND ITIRO DMURA BY C101 nnronem, 5mm Hill ATTORNEYS SCANNING-TYPE SPUTTERING MASS SPECTROMETER This invention relates to a scanning type of sputtering mass spectrometer.

By the term sputtering mass spectrometer" is meant a mass spectrometer in which a finely focused primary ion beam is directed onto the surface of a solid specimen to be analyzed and secondary ions (sputtered ions) emitted from this irradiated portion (analyzing spot) are analyzed through a mass spectroscopic system to provide the composition analysis of the specimen at said analyzed spot. This apparatus is also referred to as an ion microanalyzer.

In the case of analyzing the composition of a specimen at various points by the use of this apparatus, it is necessary to shift the irradiation spot of, the primary ion beam from one desired position to another. Especially when it is used to find the concentration distribution of a certain element in the surface of a specimen, it is necessary to scan the irradiating spot of the primary ion beam in the plane of the specimen surface and to detect only such ions of a particular mass unit, or more strictly. the mass-to-charge ration m/e, among secondary ions emitted from the surface as traced by the scanning of the primary ion beam by an ion collector.

For shifting or scanning a primary ion beam on the surface of a specimen, it can firstly be thought of a method of shifting the specimen itself. In this method, however, an extremely precise and complicated structure is necessary for precisely shifting a specimen by a minute amount (of the order of microns), and there arise technical and economic difficulties in the manufacture.

Further, the consumption of manpower and time required for its operation is very large. Therefore, it is difficult to realize a rapid and precise shift or scan of an irradiation spot by this method.

Thus, generally the specimen has conventionally been fixed and the primary ion beam has been subjected to deflection scanning. Apparatus of this type is referred to as a scanning type sputtering mass spectrometer". In such a system, since electrostatic or electromagnetic scanning of the primary ion beam can be employed, the irradiation spot can be shifted rapidly and accurately. However, the shift of the irradiation spot by the scanning of the primary ion beam, at the same time, causes the shift of the object point of the mass spectrometric system (i.e., emission point of the secondary ions). Thus, this type has a drawback in that the analyzing conditions of the mass spectrometric system are subjected to a change according to the scanning ofthe primary ion beam.

For example, in apparatus of the magnetic separation type, if the subject point is fixed which the magnitude of the accelerating voltage and the deflecting magnetic field for the secondary ions kept constant, the mass or more strictly the m/e ratio, ofthe ions collected by the ion collector is constant, but if the subject point is shifted by thescanning of the primary ion beam, the mass of ions injecting into the ion collector is changed accordingly. That is, a shift of the subject point causes a change of the starting position and hence the path of the secondary ions in the deflecting magnetic field, resulting in a shift ofthe focal point ofions ofa particular mass.

Thus, in sputtering mass spectrometers of the magnetic separation type, so far as the accelerating voltage and the deflecting magnetic field intensity are kept constant, the mass of ions striking the ion collector is changed according to the scanning of the primary ion beam and it is impossible to detect the concentration distribution of a certain element in the specimen surface accurately.

This invention aims to eliminate the above drawback in a sputtering mass spectrometer.

An object of this invention is to improve the spectroscopic accuracy of a sputtering mass spectrometer by permitting ions of only a desired mass to enter into an ion collector, irrespective of the shift of the subject point accompanied'with the scanning of the primary ion beam.

According to this invention, at least one of the accelerating voltage or the intensity of the analyzing magnetic field for the secondary ions is controlled to change in synchronism with the scanning of the primary ion beam to achieve the above-mentioned object.

Now, embodiments of the invention will be described with reference to the accompanying drawings in which:

FIG. 1 is a block diagram showing the principal structure of a scanning-type sputtering mass spectrometer according to the invention;

FIG. 2 schematically shows an example of an oscilloscope image of secondary ions emitted from .a specimen surface;

FIG. 3 is a schematic diagram showing how the mass of ions going into an ion collector changes according to the displacement of the subject point in a conventional apparatus;

FIG. 4 is a schematic diagram showing how the mass of ions .going into an ion collector is maintained constant irrespective of the shift of the subject point caused by the scan of the primary ion beam, in an apparatus according to this invention;

FIGS. 5 and 6 are block diagrams of central portions of an embodiments of the invention;

FIGS. 7a to 72 and 8a to 8e are waveforms for explaining the operation of the circuits of FIGS. 5 and 6, respectively.

FIG. 1 shows the schematic structure of a sputtering mass spectrometer of the magnetic field separation type in which ions forming the primary ion beam are emitted from an ion beam generating system 1, for example, comprising an ion gun such as a duo-plasmatron and an appropriate focusing lens and which are led to pass through a scanning system 2 comprising a pair of horizontal (X-axis) deflecting electrodes 2X and a pair of vertical (Y-axis) deflecting electrodes ZY which are respectively supplied with deflecting electrodes U, and U, generated by a scanning signal source 114. The scanning primary ion beam thus formed, strikes a specimen 3 of interest applied with a secondary ion accelerating voltage V (usually several KV) by an accelerating voltage source 4, l7. Secondary ions are accelerated through the accelerating electric field of V and then pass through an analyzing magnetic field H (usually of several Killo-Gauss) established by an analyzing electromagnet 5 of sector shape energized by current sources 6 and 16. A collector slit 7 allows only ions of a certain mass, or more strictly m/e ratio, to pass through to activate an ion collector 8. Namely, the analyzing electromagnet 5, the slit 7 and the ion collector 8 form a mass spectrometric system of the magnetic field separation type. Reference numeral 9 indicates a vacuum housing the interior of which is kept to an appropriate vacuum by suitable exhausting means (not shown).

In such a structure, a finely focused primary ion beam 1,, (usually of a diameter of from several to 10 add microns) emitted from the primary ion beam generating system i, is irradiated onto a spot (analyzing spot) A on the specimen surface to emit secondary ions (sputtering ions) 1,, the components of which correspond to those of the elements of the analyzing spot A,,. These secondary ions are accelerated by the accelerating voltage V and are deflected and mass analyzed in the magnetic field H of flux density B according to the mass of the respective ions. The m/e ratio of the ion striking the ion collector 8 through the collector slit 7 provided at a fixed position is determined by the accelerating voltage V and the magnetic flux density B by the following formula:

m/e a r BZ/V l where, r is the radius of curvature of the path of ions in the magnetic field H. The fact that the subject point (emission point of the secondary ions) and the collector slit are fixed in position means that the radius of curvature is a constant, r=r, (constant).

Thus, ions of an arbitrary mass can be selectively detected by the ion collector 8 by suitably selecting the accelerating voltage V and/or the magnetic flux density 8. When the value of V and/or B is continuously changed. the mass of ions striking the ion collector is also continuously changed, i.e., the mass spectrum of the specimen can be detected by recording the output voltage 5.. of the ion collector 8 in a recorder 15.

From this mass spectrum, the composition of elements at the analyzing spot A can be determined.

On the other hand. when the primary ion beam I, is scanned in the plane of the specimen surface by the ion beam-scanning system 2 (this is accompanied with the change in the radius of curvature of ions detected by the ion collector), and the accelerating voltage and/or the magnetic field is arranged to change so that ions of a certain mass always strike the ion collector, the emission distribution of ions of a certain mass in the specimen surface and hence the concentration distribution of a component element(s) of a certain mass can be detected. Provided that the output voltage of the ion collector is proportional to the concentration of a particular element in the specimen surface, a brightness-modulated image (secondary ion image) of the concentration distribution of a particular element in the specimen surface can be represented on the image screen 13 ofa cathode-ray tube oscilloscope 10 by supplying the output voltage E of the ion collector 8 to the brightness modulation terminal 11 of said oscilloscope I and thus scanning the modulated image-forming electron beam e in synchronism with the scanning of the primary ion beams I,,. It will be apparent that the brightness variation in the image screen represents the concentration variation in the specimen surface. For the synchronous scanning, the pair of horizontal (X-axis) deflecting electrodes 12X and the pair of vertical (Y- axis) deflecting electrodes 12! of the CRT oscilloscope are respectively supplied with voltages U, (which is proportional to U,.) and U, (which is proportional to Uy) by the scanning signal source 14.

FIG. 2 shows a model of such a secondary ion image in which the concentration variation of a particular element in the specimen surface is represented as the brightness variation of regions 5,. S S and 5,. It is to be noted that the secondary ion image can represent the concentration distribution of a particular element in the specimen surface only when it is assured that the mass of ions striking the ion collector does not change.

However. according to the conventional system, even when the primary ion beams is scanned on a specimen surface, the accelerating voltage V and the magnetic flux B are kept constant and yet the secondary ion image formed by scanning the primary ion beam is treated as a representation of the concentration distribution of a particular element. In other words, variations in position of the subject point have been neglected.

FIG. 3 illustrates the principle of a conventional system in which a specimen 3 and a collector slit 7 are disposed in position as is shown in the figure with respect to the center 0 of curvature of the standard path (0,, of ions in the sector shaped magnetic field H established by analyzing electromagnet 5. The standard path m, has a radius of curvature r and the accelerating voltage V and the magnetic flux density B are kept constant (V=v,., B=B,). When the primary ion beam I, is scanned in the X-axis direction with a scanning width X, the irradiation spot shifts on the specimen surface from A, to A with a center at A and this will cause the displacement of the subject point of the mass spectrometer in the Z-axis direction to a width 2,. As among the secondary ions emitted from point A,,, ions of a particular mass (m/e ratio) defined by equation I take the standard path w, (radius of curvature r,) to focus onto spot 11,, ions of the same mass emitted from points A, and A take the paths w, and a), (radius of curvature being both constant r,.) to focus onto spots a and a; respectively. That is, if the emission position (i.e., subject position) is different, even ions of the same mass take different paths in the deflecting magnetic field and subsequently focus to different positions. Thus ions focusing on spots different from 0 cannot be correctly collected by the ion collector through the slit 7 having a slit width d. In another aspect, when the irradiation spot is shifted to a position A, or A different from A ions of some different mass than the desired mass are inevitably collected by the ion collector 8 to provide a detection result not true to the desired one.

This invention eliminates the above drawback by changing at least one of the accelerating voltage V and deflecting the magnetic flux density B in synchronism with the scanning of the primary ion beam so as to let ions ofonly the same mass go into the ion collector.

FIG. 4 illustrates the principle of this invention. When the irradiation spot of the primary ion beam is located at point A,,, ions emitted from there and having a certain mass take the standard path w,,( radius of curvature being Fn) to focus onto spot a,,. When it is shifted to points A, or A ions of said mass take path in, (radius of curvature r r,.) or w, (radius of curvature r r to focus on the same spot a, as before.

As is apparent from the figure, according to the invention, the accelerating voltage V and/or the flux density B of the analyzing magnetic field is varied in synchronism with the scan of the primary ion beam to compensate the displacement of the irradiation spot so as to always focus ions of a particular mass on a fixed spot. By such an arrangement, the ion collector always receives ions of a constant mass, irrespective of the scan of the primary ion beam.

Now, consideration will be given to the relation among the rates of variation for the mass of ions Am/m, the radius of curvature of the path of ions Ar/r, the accelerating voltage AV/ V and the magnetic flux density AB/B. The differentiation of equation l approximately gives the following relation; Am/ m =2Ar/H-2AB/B-A V/V (2) For keeping the mass of ions striking the ion collector constant, the condition Am/m=0 should be satisfied.

Thus, if the magnetic flux B is kept constant (i.e., AB=0) the condition; AV/V=2Ar/r (3) results in A"I/II|=O. And if the accelerating voltage V is kept constant (AV=0) the condition of AB/B=-Ar/r provides the result of Am/m=0.

A displacement of the subject point of the mass spectrometer in the Z axis direction by AZ corresponds to a variation of the radius ofcurvature r of the ion path by AZ, namely Ar=AZ. Further, displacement AZ is proportional to a displacement AX of the irradiation spot which is further proportional to the deflecting voltage U," Thus, in the region where AX is small, the following relations can be established from equation (3) and (4),AVaAraU, (5) ABa-A.r a-U, ..(6)

Namely, for keeping the mass of ions striking the ion collector constant, irrespective of the scanning of the primary ion beam, with the magnetic flux density 8 kept constant, it is sufficient to vary the accelerating voltage V in proportion to the deflecting voltage U,. On the other hand, when the accelerating voltage V is kept constant, it is sufficient for keeping Am/m=, to vary the magnetic flux density 8 in proportion, but in opposite polarity to the deflecting voltage U,.

FIG. 5 shows an example ofa circuit for the case of a constant accelerating voltage V=V,. in which the magnetic flux density 8 is varied in the form of B=B,+AB. In the analyzing electromagnets 5A and 58, primary windings 18A, 18B and auxiliary windings 19A, 19B are respectively provided. A constant current I is allowed to flow through the primary windings 18A and 188 by a primary current source 6 to establish a constant magnetic flux 8,. Through the auxiliary windings 19A and 198, a current Al, the magnitude of which is proportional to the deflecting voltage U in opposite direction is allowed to flow from the scanning signal source 14 to establish a magnetic flux AB in proportion to the auxiliary energizing current Al. Thus, the total magnetic flux becomes B+AB.

FIGS. 7a to 7e illustrate the operation of the circuit of FIG. 5, in which FIG. represents the variation of the deflecting voltage U, for the primary ion beam, FIGS. 7b and 7c displacement distances AX and AZ of the irradiation spot and the subject point respectively, FIG. 7d the auxiliary energizing current AI, and FIG. 70 the variation Ar of the radius of curvature of the ion path in the magnetic field. As is apparent from this figure, the displacement AZ of the subject point is proportional to the deflecting voltage U but since the auxiliary current AI is variedin proportion but in opposite polarity to the deflecting voltage U, the radius of curvature varies to cancel the displacement of the subject point, AI=AZ. Thus, the ion collector can receive only the ions of a particular mass.

Next, FIG. 6 shows another embodiment of a circuit for the case of a constant magnetic flux where B=B,., in which the accelerating voltage is varied in the form of V =V,.+A V. The circuit comprises a series connection of a main accelerating voltage source 4 and an auxiliary accelerating voltage source 17. The main voltage source 4 supplies a constant voltage V,. and the output voltage AV of the auxiliary voltage source 17 is varied in proportion to the deflecting voltage from the scanning signal source 14. Thus, an accelerating voltage of V=V +AV is applied to the specimen 3. FIGS. 8a to Sc illustrates the operation of the circuit of FIG. 6. The auxiliary accelerating voltage AV (FIG. 8a) is varied in proportion to the deflecting voltage U, (FIG. 8a) to vary Ar in proportion to U,, thus cancelling A2 with Ar.

In the above embodiments, the accelerating voltage V and/or the magnetic flux density B is varied corresponding to the Xaxis deflecting voltage U, This corresponds to the case where the scanning in the Y-axis direction does not produce any displacement of the subject point in the Z-axis direction. If it does not, the voltage V and/or the flux B should naturally be varied in correspondence to the Y-axis deflecting voltage U In a mass spectrometer of the magnetic field separation type, displacement of the subject point in a direction perpendicular to the deflection plane of ions is negligible and cause no problems, therefore, it is usually sufficient to vary either V or 8 corresponding to one deflecting voltage causing the displacement of the subject point in the Z-direction.

Further, in FIG. 1, it will be apparent that one of the auxiliary accelerating voltage source 17 and the auxiliary current source 16 is not necessary for a particular type of apparatus. It will be convenient for changing the mass ofions to be detected by the ion collector if the output voltage ofthe main accelerating voltage source or the output current I, of the main energizing current source 6 is arranged to be variable.

As is apparent from the foregoing description, according to the present invention, the mass of ions striking an ion collector can be kept constant by a simple arrangement of varying at least one of the accelerating voltage V and the flux density B of the analyzing magnetic field for the secondary ions in synchronism with the scanning of the primary ion beam. Thus this invention is particularly useful for improving the spectroscopic accuracy when the concentration distribution of a particular element on a specimen surface is to be measured.

We claim:

1. in a scanning-type sputtering mass spectrometer including a primary ion generator having means for irradiating a finely focused primary ion beam on a surface of a solid specimen to be analyzed, scanning means for scanning said primary ion beam on the surface of said specimen to generate a secondary ion beam, electromagnetic means for generating a magnetic field to analyze said secondary ion beam emitted from said specimen as a result of irradiation of said primary ion beam thereon. an ion collector selectively positioned for collecting the analyzed ion beam derived from said electromagnetic means, an accelerating voltage source applying an accelerating voltage to said specimen, and an exciting current source supplying an exciting current to said electromagnetic means, the improvement comprising compensating means for varying at least one of the accelerating voltage and the exciting current synchronously with the scanning of said primary ion beam so as to direct ions of a predetermined mass to strike said ion collector irrespective of the position of the primary ion beam on said specimen.

2. A sputtering mass spectrometer according to claim I, wherein said scanning meansis an electrostatic deflecting device, and said compensating means varies one of the acceleratmg voltage and the exciting current in proportion to the voltage energizing said electrostatic deflecting device.

3. A sputtering mass spectrometer according to claim I, wherein said compensating means varies the accelerating voltage synchronously with the scanning ofsaid primary ion beam.

4. A sputtering mass spectrometer according to claim 1, wherein said compensating means varies the exciting current synchronously with the scanning of said primary ion beam.

5. A sputtering mass spectrometer apparatus according to claim I, further comprising means for displaying the image of secondary ions through the output voltage of said ion collector. 

2. A sputtering mass spectrometer according to claim 1, wherein said scanning means is an electrostatic deflecting device, and said compensating means varies one of the accelerating voltage and the exciting current in proportion to the voltage energizing said electrostatic deflecting device.
 3. A sputtering mass spectrometer according to claim 1, wherein said compensating means varies the accelerating voltage synchronously with the scanning of said primary ion beam.
 4. A sputtering mass spectrometer according to claim 1, wherein said compensating means varies the exciting current synchronously with the scanning of said primary ion beam.
 5. A sputtering mass spectrometer apparatus according to claim 1, further comprising means for displaying the image of secondary ions through the output voltage of said ion collector. 